Setting max length in summernote

you can make it with callback object and preventDefault function.

This sample is with 400 limit.

  function registerSummernote(element, placeholder, max, callbackMax) {
    $(element).summernote({
      toolbar: [
        ['style', ['bold', 'italic', 'underline', 'clear']]
      ],
      placeholder,
      callbacks: {
        onKeydown: function(e) {
          var t = e.currentTarget.innerText;
          if (t.length >= max) {
            //delete key
            if (e.keyCode != 8)
              e.preventDefault();
            // add other keys ...
          }
        },
        onKeyup: function(e) {
          var t = e.currentTarget.innerText;
          if (typeof callbackMax == 'function') {
            callbackMax(max - t.length);
          }
        },
        onPaste: function(e) {
          var t = e.currentTarget.innerText;
          var bufferText = ((e.originalEvent || e).clipboardData || window.clipboardData).getData('Text');
          e.preventDefault();
          var all = t + bufferText;
          document.execCommand('insertText', false, all.trim().substring(0, 400));
          if (typeof callbackMax == 'function') {
            callbackMax(max - t.length);
          }
        }
      }
    });
  }


$(function(){
  registerSummernote('.summernote', 'Leave a comment', 400, function(max) {
    $('#maxContentPost').text(max)
  });
});
    

<!-- include libraries(jQuery, bootstrap) -->
<link href="https://netdna.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.css" rel="stylesheet">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.js"></script>
<script src="https://netdna.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.js"></script>

<!-- include summernote css/js -->
<link href="https://cdnjs.cloudflare.com/ajax/libs/summernote/0.8.9/summernote.css" rel="stylesheet">
<script src="https://cdnjs.cloudflare.com/ajax/libs/summernote/0.8.9/summernote.js"></script>

<div class="container">
  <div class="row">
    <div class="col-xs-12">
      <div class="summernote"></div>
    </div>
    <div class="col-xs-12 text-right">
      <span id="maxContentPost"></span>
    </div>
  </div>
</div>

Based on Angel Fraga Parodi's answer I added some more keyCodes that should always be allowed like delete, arrow keys or ctrl+x/ctrl+c. Also the code for pasting wasn't working (anymore). Here's an updated version:

<div class="summernote" ></div>
<h5 id="maxContentPost" style="text-align:right"></h5>
<script>
        $(document).ready(function () {
            $('.summernote').summernote({
                toolbar: [
                  ['style', ['bold', 'italic', 'underline', 'clear']]
                ],
                placeholder: 'Leave a comment ...',
                callbacks: {
                    onKeydown: function (e) { 
                        var t = e.currentTarget.innerText; 
                        if (t.trim().length >= 400) {
                            //delete keys, arrow keys, copy, cut, select all
                            if (e.keyCode != 8 && !(e.keyCode >=37 && e.keyCode <=40) && e.keyCode != 46 && !(e.keyCode == 88 && e.ctrlKey) && !(e.keyCode == 67 && e.ctrlKey) && !(e.keyCode == 65 && e.ctrlKey))
                            e.preventDefault(); 
                        } 
                    },
                    onKeyup: function (e) {
                        var t = e.currentTarget.innerText;
                        $('#maxContentPost').text(400 - t.trim().length);
                    },
                    onPaste: function (e) {
                        var t = e.currentTarget.innerText;
                        var bufferText = ((e.originalEvent || e).clipboardData || window.clipboardData).getData('Text');
                        e.preventDefault();
                        var maxPaste = bufferText.length;
                        if(t.length + bufferText.length > 400){
                            maxPaste = 400 - t.length;
                        }
                        if(maxPaste > 0){
                            document.execCommand('insertText', false, bufferText.substring(0, maxPaste));
                        }
                        $('#maxContentPost').text(400 - t.length);
                    }
                }
            });
        });
</script>